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Civil Engineering/ETABS static EQ RSA and Mass source


1- what is the difference between eigenvectors and Ritz vectors while defining response spectrum case in etabs
2- why we define a static load case of type quake, then we define a response spectrum function and response spectrum case, what is the purpose of each one and what is the difference between them
3- could you plz explain the difference between the three options presents in the mass source definition in etabs

Hi Zena,

Most of the questions you ask are answered in the CSI knowledge base:


2. You need  static cases first to determine maximum base shears, so that you can then scale the RSA factors to arrive at base shears within 90% of static. For RSA you first need to define the function (e.g. based on UBC or any other code) and then you define the RSA cases based on directions and factors applied to the function. Refer to:

3. The 3rd option is a combination of the first two. The first option considers mass from self (without a factor)+ any specified mass (such as tuned mass dampers, etc). The second one works out the mass from self weight and loads, but you can apply factors to these- e.g. 0.95 to self to account for overlaps in walls and slabs, etc. Check this out:

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Arshad Khan


I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.


Employment history: 40 years in Construction and consultancy in the UK, Kenya, Tanzania, Uganda, Ethiopia, Eritrea, South Africa, Somalia, Zambia, Austria, Bahrain, Kuwait, Doha and the U.A.E.

.Fellow of Institution of Structural Engineers (UK) .Fellow of Institution of Civil Engineers(UK) .Member of the Institution of Engineers, Kenya .Registered Engineer, ERB, Kenya .Member of the Architectural Association of Kenya (Engineers Chapter) .Chartered Engineer (UK)

•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi •2013: IEK International Conference, Kisumu, Kenya

BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to 2010)

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